In many applications, it must be possible to position power take-off elements, which are moved by a positioning drive, very precisely, for example tables or carriages of machine tools or measurement machines, in order to obtain correspondingly great precision in machining or measuring or testing of a work piece. In the production of such a positioning drive, the movement-transferring mechanism demonstrates elasticity, production tolerances, friction, and play or slack. On the basis of these insufficiencies, imprecisions or errors can come about in the positioning of the power take-off element.
The play in the mechanical transfer within the positioning drive all the way to the power take-off element can furthermore lead to a delayed movement of the power take-off element if a positioning motor that is being controlled must first overcome the play before its movement is transferred to the power take-off element. In the case of highly dynamic applications, problems or errors can occur during positioning, such as, for example, oscillations or overshooting behavior, which make it necessary to reduce the amplification and thereby the dynamics of the position regulator.
It is therefore desirable to eliminate the play in the kinematic transfer chain from a positioning motor all the way to the power take-off element.
In order to solve this problem, it is known to eliminate a play in the kinematic transfer chain by means of elastic preload elements. It is disadvantageous, in this regard, that the preload force of a mechanical preload element acts at every point in time, and this has a negative influence on the efficiency of the positioning drive. Devices for being able to optionally activate and deactivate such a mechanical preload element are complicated in terms of design and require additional actuators.
In order to avoid the disadvantages of a mechanical preload element, it is also known to drive a power take-off element by means of a positioning drive having two servomotors, to which a moment for preload of the mechanical system can be applied, one against the other. Such a positioning drive is known, for example, from DD 279432 A1. There, two servomotors are operated, regulated with a control loop, which has an overriding position control loop. A speed of rotation control loop is subordinated to the position control loop, and, in turn, a current control loop is subordinated to the speed of rotation control loop. In the case of servomotors having field-oriented current regulation, the motor moment can be changed by means of influencing the torque-forming motor current (armature transverse current), which moment behaves proportional to the motor current. Accordingly, a tensioning reference value transducer is additionally present, which modifies the reference values for the motor currents of the two servomotors in such a manner that these each maintain a motor moment that is directed opposite the other. As a result, the mechanical system is tensioned and the power take-off element can be positioned without play and precisely.
Proceeding from this, it can be seen as the task of the present invention to create a positioning drive as well as a method, which can be implemented with simpler means.